1. Field of the Invention
The present invention relates to a head used for thermally-assisted magnetic recording that irradiates near-field light to a magnetic recording medium and records data by decreasing an anisotropic magnetic field of the magnetic recording medium, and to a head gimbal assembly and a magnetic recording device using the head.
2. Description of the Prior Art
In the field of magnetic recording using a head and a medium, further performance improvements of thin film magnetic heads and magnetic recording media have been demanded in conjunction with a growth of high recording density of magnetic disk devices. For the thin film magnetic heads, composite type thin film magnetic heads with a configuration in which a magnetoresistive (MR) element for reading and an electromagnetic conversion element for writing are laminated are currently widely used.
The magnetic recording medium is a discontinuous medium in which magnetic microparticles gather and each of the magnetic microparticles has a single magnetic domain structure. In this magnetic recording medium, a single recording bit is configured with a plurality of magnetic microparticles. Therefore, in order to increase recording density, asperities at a border between adjacent recording bits need to be reduced by decreasing the size of the magnetic microparticles. However, reducing the magnetic microparticles in size leads to a decrease in the volume of the magnetic microparticles, resulting in a decrease in the thermal stability of magnetizations in the magnetic microparticles.
As a countermeasure against this problem, increasing magnetic anisotropy energy Ku of the magnetic microparticles may be considered; however, the increase in Ku causes an increase in an anisotropic magnetic field (coercive force) of the magnetic recording medium. On the other hand, the upper limit of the writing magnetic field strength of the thin film magnetic head is substantially determined by a saturation magnetic flux density of a soft magnetic material configuring a magnetic core in the head. As a result, when the anisotropy magnetic field of the magnetic recording medium exceeds an acceptable value determined from the upper value of the writing magnetic field strength, it becomes impossible to write. Currently, as a method to solve such a problem of the thermal stability, a so-called thermally-assisted magnetic recording method has been proposed in which, while a magnetic recording medium formed of a magnetic material with large Ku is used, the magnetic recording medium is heated immediately before the application of the writing magnetic field to reduce the anisotropic magnetic field and thereby the writing is performed.
For the thermally-assisted magnetic recording method, a method using laser light is common as a method for heating the magnetic recording medium. More specifically, there are a method (direct heating) in which laser light is guided to the vicinity of a recording portion of a magnetic recording medium by an optical waveguide or the like to heat the magnetic recording medium and another method (near-field light heating) in which laser light is converted to near-field light to heat the medium.
As an example of the direct heating, JP Patent Laid-Open H10-162444 discloses a technology that records extremely minute magnetic domain signals to an optical magnetic disk using a solid immersion lens.
Also as examples of the near-field light heating, JP Patent Laid-Open 2001-255254 discloses an optical recording technology using a near-field light probe configured with a metal scatterer in the shape of circular cone, triangle or the like formed on a substrate and a film such as a dielectric body formed in the vicinity of the scatterer. And also, JP Patent Laid-Open 2004-158067 discloses a technology in which a scatterer configuring a near-field light probe is disposed in a manner of contacting a main pole of a single pole writing head for perpendicular magnetic recording such that the scatterer is disposed perpendicular to the recording medium.
Note, near-field light is one type of a so-called electromagnetic field that is formed in the vicinity of substances, and has a property that can ignore a diffraction limitation due to a wavelength of light. By irradiating light having identical wavelength to a minute structure body, it becomes possible to form a near-field depending on the scale of a minute structure body and even to focus light to a minimum region of several tens of nanometers.
As a specific method for generating near-field light, a method using a so-called plasmon antenna, which is a near-field light probe formed of a metal piece that generates near-field light from plasmon excited by light, is generally known.
In the above-described method using the plasmon antenna, near-field light is generated by direct irradiation of light to the plasmon antenna (for example, JP Patent Laid-Open No. 2010-80044, JP Patent Laid-Open No. 2010-49781, or the like); however, the conversion efficiency from the irradiated light to the near-field light is low with this method. In other words, most of the energy of the light irradiated to the plasmon antenna reflects off a surface of the plasmon antenna or is converted to thermal energy. Since the size of the plasmon antenna is set to the wavelength of the light or less, the volume of the plasmon antenna is small.
As a result, a temperature increase due to heat generation of the plasmon antenna becomes extremely large, the plasmon antenna may be easily diffused/melted, and a negative effect that the plasmon antenna cannot play its role may occur.
As disclosed in, for example, US 2010/0103553, a technology is proposed in which light propagating through a waveguide is coupled with a near-field light generating portion (plasmon generator: PG) with a buffer layer therebetween in a surface plasmon polariton mode to excite surface plasmon on the plasmon generator without directly irradiating light to the plasmon antenna.
In the proposal, the plasmon generator includes a near-field light generator that is positioned on a surface opposing the magnetic recording medium and that generates the near-field light. In the technology, when the light propagating through the waveguide totally reflects off an interface between the waveguide and the buffer layer, evanescent light penetrating into the buffer layer is generated, the evanescent light couples to collective oscillation of charge, which is surface plasmon, on the plasmon generator, and the surface plasmon is excited on the plasmon generator. The surface plasmon excited on the plasmon generator propagates to a near-field light generator through a propagation part (such as an edge), and thereby near-field light is generated from the near-field light generator positioned on the surface opposing the magnetic recording medium.
According to this technology, since the light propagating through the waveguide is not directly irradiated to the plasmon generator, it is possible to prevent an excessive temperature increase. This type of element is occasionally referred to as a surface evanescent light coupling type near-field light generating element.
Meanwhile, in the thermally-assisted recording element, temperature increases not only in the magnetic recording medium, which is a target to heat, but also in the thermally-assisted recording element occur. As a result, selective thermal expansion occurs in the vicinity of the plasmon generator. Specifically, metals such as, for example, Au, Ag, Cu or the like, which are used as materials for a plasmon generator with high efficiency, have a higher thermal expansion rate compared to surrounding materials for a dielectric body that is used as a cladding materials and a core material, a pole, or the like.
Furthermore, the metals such as Au, Ag, Cu or the like, which are used as the materials for the plasmon generator, have a lower hardness compared to the materials positioned in its vicinity. Moreover, the plasmon generator is configured such that locally excessive heating can be suppressed; however, its volume is larger compared to the volume of a conventional plasmon antenna. Therefore, it can be said that, with the configuration, effects due to the volume expansion of the plasmon generator are more likely to occur when the plasmon generator is entirely heated.
In this way, with the plasmon generator that is formed of Au, Ag, Cu or the like having a property of a high thermal expansion rate and a low hardness and that occupies a predetermined volume, a problem that the plasmon generator itself projects from an ABS, which is a so-called air bearing surface, may occur due to the temperature increase. The plasmon generator that is projected from the ABS, which is the so-called air bearing surface, as described above may collide with the magnetic recording medium, so that a variety of negative effects such as a loss of the plasmon generator, a decrease in the flying stability or the like may occur.
The present invention was invented based on the above-described condition. The objective of the present invention is to provide a thermally-assisted magnetic recording head that can suppress a chronological degradation of output to achieve thermally-assisted recording having a high and long-term reliability by suppressing the projecting of the plasmon generator due to the temperature increase from the ABS, which is the air bearing surface, and to provide a head gimbal assembly and a magnetic recording device that are configured with the above-described head.